1. Field of the Invention
The present invention relates to a magnetic bearing apparatus provided with a touchdown bearing made of a pair of roller bearings and a pair of corrugated plate-shaped damper members inserted into an annular gap formed between the touchdown bearing and its retainer member and to a vacuum pump having the magnetic bearing apparatus, and more particularly to an improvement in durability of a touchdown bearing and corrugated damper members for absorbing shock upon the touchdown and suppressing to a sufficiently low level a vibratory rotational frequency of a rotor relative to a rotational frequency of the rotor.
2. Background Information
A magnetic bearing apparatus provided at least with a rotor shaft, a radial magnetic bearing for supporting the rotor shaft in a radial direction, a thrust magnetic bearing for supporting the rotor shaft in an axial direction and a touchdown bearing is adopted in, for example, a vacuum pump such as a turbo molecular pump and has been extensively practiced. The above touchdown bearing is a protective bearing for receiving the above rotor shaft in case of emergency like a breakdown of the magnetic bearing and is composed of, for example, a pair of roller bearings arranged at a lower end portion of the above rotor shaft. Such a magnetic bearing apparatus and the turbo molecular pump provided with this are disclosed in JP-A-10-89284, JP-A-63-239397 and the like.
Also, the corrugated plate-like damper member adopted in such a magnetic bearing apparatus is a member for suppressing a vibratory rotation, i.e., a swivel motion and simultaneously absorbing the shock of the touchdown when the rotor including the rotor shaft touches down to the touchdown bearing. This corrugated plate-like damper member functions as three members of a spring, a damper and a mechanical stop as one member as disclosed in JP-B-7-103894, and is, for example, a corrugated strip steel plate as shown in FIG. 6.
In FIG. 5, the corrugated plate-like damper member is composed of a pair of corrugated strip steel plates 8a and 8b inserted into an annular gap G formed between outer races of a pair of roller bearings 4a and 4b constituting the touchdown bearing 4 and an inner circumferential surface of a retainer member 9 of the touchdown bearing. The frequency f=(k/m)xc2xd determined by the rigidity k of the corrugated strip steel plates 8a and 8b and the rotor mass m is identified with the vibratory rotational frequency of the rotor upon the touch down. The collision energy E upon the touchdown of the rotor is in proportion to the second powered value of the frequency f, i.e., (k/m). From these relationships, it will be understood that the smaller the rigidity k, the larger the effect of the corrugated strip steel plates 8a and 8b will become as the damper member. In order to reduce the rigidity k, it is available to reduce the thickness t of the corrugated strip steel plates 8a and 8b, for example, but the function of the corrugated strip steel plates 8a and 8b as the stop is degraded as the thickness t is decreased.
In order to cause the corrugated strip steel plate 8 to serve as a stop with a rigidity to some extent against the shock upon the touchdown of the rotor, the pitch p of the waveform, the height h under the non-load condition and the thickness t of the corrugated strip steel plate 8 could not be reduced as desired, as a result of which the width B has to be reduced. For this reason, the height H of the roller bearing would be twice greater than the width B of the corrugated strip steel plate 8 or more. In other words, in some cases, the corrugated strip steel plate 8 having a width that is less than half the height H of the roller bearing must be used. The corrugated strip steel plates 8a and 8b that have small pitch p, height h and thickness t and a width that is less than half the height H of the roller bearing are provided with a small retaining force. Accordingly, in the case where such corrugated strip steel plates 8a and 8b having the smaller width are inserted into the annular gap G, due to the vibration caused by the use for a long period of time, the upper corrugated strip steel plate 8a is offset downwardly so that it is brought into contact with the lower corrugated strip steel plate 8b. As a result, in some cases, the first rotary bearing 4a is kept free by width xcex4 in the radial direction to be brought into contact with the rotor to generate abnormal noise or abnormal frictional wear. Incidentally, the width xcex4 is the width of the annular gap G. In such a condition, the corrugated plate-like damper member could not exhibit the inherent function and could generate abnormal vibration or swing in the vacuum pump provided with the magnetic bearing apparatus or the magnetic bearing to bring about a breakdown in the apparatus as a whole.
The present invention provides a magnetic bearing apparatus provided at least with a rotor shaft, a radial magnetic bearing for supporting the rotor shaft in a radial direction, a thrust magnetic bearing for supporting the rotor shaft in an axial direction, a touchdown bearing composed of a pair of roller bearings arranged at a lower end portion of the above rotor shaft, and a pair of corrugated plate-shaped damper members inserted into an annular gap between the touchdown bearing and its retainer member, and to a vacuum pump provided the magnetic bearing apparatus. An object of the present invention is to securely maintain the pair of corrugated plate-shaped damper member in a predetermined position.
In order to solve the above-noted problems, there is provided a magnetic bearing apparatus provided at least with a rotor shaft, a radial magnetic bearing for supporting the rotor shaft in a radial direction, a thrust magnetic bearing for supporting the rotor shaft in an axial direction, a touchdown bearing composed of a pair of roller bearings arranged to surround a lower end portion of the rotor shaft, a pair of corrugated plate-shaped damper members inserted into an annular gap disposed between the touchdown bearing and its retainer member and positional offset preventing means for preventing positional offset of the corrugated plate-shaped damper members disposed in the annular gap.
Then, a metal thin plate interposed between the pair of corrugated plate-like damper members is used as the positional offset preventing means of the corrugated plate-like damper member.
Also, an annular convex portion formed in an inner circumferential surface of the retainer member is used as the positional offset preventing means of the corrugated plate-like damper member.
Further, an annular concave portion formed in an inner circumferential surface of the retainer member is used as the positional offset preventing means of the corrugated plate-like damper member.
Furthermore, when the positional offset preventing means of the corrugated plate-like damper member is constructed, a thickness of a metal thin plate, a sum (T+t) of a height of the annular convex portion or a depth of the annular concave portion T and a thickness t of a corrugated strip steel plate is 0.8 to 1.3 times of a width xcex4 of the annular gap.
The present invention is also directed to a vacuum pump having the magnetic bearing apparatus according to the present invention.